Numerous safety devices and arrangements have been developed to protect a machine operator from injury during operation of various types of work fabricating devices, such as machine tools, rivet staking punches, laser welding devices, cutters. The usual arrangements include a pair of spaced buttons which must be simultaneously depressed by the machine operator in order to actuate an interlock facility that permits the operation of the work fabricating device. These interlocks take many forms and may be electrical, mechanical, pneumatic or a combination thereof. In other situations, mechanical barriers are moved into position to prevent the positioning of the operator's hands in the path of the work fabricating device. Further, these safety facilities are usually associated with a work holder that moves along fixed guideways into and out of work fabricating position.
Considering some specific examples of the prior art, there is shown in U.S. Pat. No. 3,953,770 issued Apr. 27, 1976 to J. Hayashi, a safety arrangement wherein a high frequency electric field is formed around a machine operator standing on a wire mat having high frequency power applied thereto. A high frequency sensing antenna is positioned in the vicinity of a machine cutting tool to detect the presence of any part of the operator's body by sensing the presence of the electric field, which sensing is accompanied by the generation of a signal that functions to operate a switch to interrupt the cycling of the machine.
Other art that may be of interest with respect to the present invention includes U.S. Pat. No. 3,787,732 issued Jan. 22, 1974 to W. A. Larson, which patent discloses an electric switch comprising a pair of spaced electrodes that may be bridged by an operator's finger to control the operation of an output electronic switch. More particularly, the bridging of the spaced electrodes by the operator's finger connects the skin resistance of the finger into an input circuit to change the impedance of this circuit to initiate operation of a pair of tandem connected amplifiers to produce a switched output.
In U.S. Pat. No. 3,908,625 to Colglazier et al. dated Sept. 30, 1975, there is illustrated a touch actuated electronic switch having a pair of spaced electrodes for controlling the operation of a differential sensor and an amplifying circuit to produce a switched output. This switch circuit depends for its operation upon an operator touching either one of the two electrodes to include the capacitance of the operator's body in the input circuit for the differential sensor, which inclusion results in a change in input impedance that causes an amplifying circuit to produce the desired switched output.
There is still a need for a safety device that permits a machine operator to freely advance a slide-like work holder on a work table into a number of work positions relative to a machine tool while at the same time insuring that the machine operator's hands are not in position to be struck by a moving machine tool as it fabricates a workpiece.
A problem in providing safety facilities also exists where it is desired to perform a number of work operations on a workpiece which has a number of different elevations upon which work operations are to be performed.